More Sample Queries

The SELECT and FROM blocks tell the database to look in the
specObj table (which contains spectroscopic information) and return
the redshift (z), the right ascension, the declination, and
the (final, or "best") object ID. The WHERE block tells the
database to look only at galaxies with redshifts above 0.3 and no
known problems with the redshift determination (zWarning = 0).

Logical Operators

In this query, the word AND appears between all the characteristics in the
where block. The query includes three characteristics - matching objects
must be galaxies, they must have redshift > 0.3, and there must be
no warnings about the redshift determination. All three of these characteristics must be
met by a given record for the search to return that record.

AND is just one of three logical operators used by SQL; the
others are OR and NOT. The meanings of the three
logical operators are given in the table below:

Logical Operator

Meaning

AND

all characteristics met

OR

at least one characteristic met

NOT

characteristic not met

You can combine the logical operators using parentheses. For example, for
the characteristics A, B, and C, A AND (B OR C) means that either
characteristics B or C, as well as characteristic A, must be met for records
to match. What does A AND (NOT B) mean?

The query below is like the first query except that it will return both
galaxies and quasars. The OR statement in
the where block causes the query to return both.

Mathematical and Conditional Operators

SQL includes a variety of mathematical operators that perform
math functions in a query. You can use multiplication, division, addition and subtraction. SQL
uses the same symbols for these operators that most other computer languages
use: + for addition, - for subtraction, * for multiplication, and / for
division. Other mathematical operators include COS(x), which returns the cosine
of column x, or SQRT(x), which returns the square root of column x.
Here is a
complete list of SQL's mathematical operators.

You can also use the conditional operators summarized below.

Conditional

Meaning

=

equal to

>

greater
than

<

less
than

>=

greater
than or equal to

<=

less
than or equal to

<>

not
equal to

Let's say you wanted to find a list of very blue
stars. In astronomy, color is defined by the differences in
magnitudes (see the
SkyServer Color Project
for more information). In the SDSS's system of measurement,
blue stars have u-g < 0.5. So to find blue stars, you might write a
query like the following:

This query returns the first 10 photometric objects
(in the PhotoObj table) that are labeled as stars (denoted by type = 6)
where u - g < 0.5. You could further refine your query
by searching for a certain ra and dec. Or you could limit
yourself to bright stars by specifying a magnitude limit such as
modelMag_r < 17.

Try It!

Try pressing the buttons "Query 1," "Query 2," and "Query 3" below. These
buttons will make the three queries above (in the purple boxes) appear in the
query window. Press Submit to execute the queries.
Write down a few of the ObjIDs, then use SkyServer's
Explore
tool to examine the objects. Do they have the characteristics
you requested in the where block? What kinds of astronomy questions
do you think you could answer by studying these objects?

Click Next to get some more practice in writing SQL queries.

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Enter your SQL query in the text box.
The query is limited to 10 minutes and 100,000 rows.